Go modules and Dockerfile support

[AngerM]

Related issue: #739

This moves away from the vendored dependencies and switches to go modules, as well as adding a Dockerfile that contains the gh-ost executable.

The only side-effect was the RelaxedParserMode that you guys had made changes for directly in your vendored version (as opposed to forking it). I dropped the one place that was used. If it's still desired, it would be best to fully fork the gcfg package and make the changes there.

[zmoazeni]

Hey @AngerM did you see my comment at #739 (comment) ?

There's a few dependencies that have changes that aren't upstream.

Are we able to ignore those changes and move to upstream? Some of them seemed important.

[AngerM]

The only one that I noticed was the gcfg RelaxedParserMode. For the rest of the packages they seemed to accept upstream versions just fine. We admittedly haven't done extensive testing with this version (or any version) yet in production.

[timvaillancourt]

@AngerM go modules has been implemented in #935

Can you update the PR if it's still necessary?